18 Questions
What system unloads the compressor during start and up to 13,000 RPM in the Allison T56 engine?
Compressor Bleed Valve System
At what percentage of engine speed does the compressor bleed valve system operate up to?
94%
What happens when the operator selects low speed ground idle or engine shutdown?
Compressor bleed valves open
What is responsible for controlling the compressor bleed valve system?
Speed Sensitive Valve
During which engine speed range is the engine governed when low speed ground idle is selected?
69% - 75%
What effect does the spring tension have on the main actuator piston in the bleed band operation?
Pushes the piston down
What happens to the compressor bleed air valves when the engine is running below 94% RPM?
They are forced closed.
How does the speed sensitive valve flyweights influence the compressor bleed valve system operation?
They direct 14th stage air onto the bleed air valve piston heads.
What function does the compressor bleed surge control system serve during low-speed operations including engine start?
Relieving the engine of excess air.
At what stage does air flow inward through the compressor bleed valve during engine cranking?
Inlet of the rear compressor.
Why is the compressor bleed surge control system used on multi-spool engines?
To relieve excess air at low engine RPM.
What drives the front compressor by the time the engine starts?
Second, third, and fourth stage turbines.
What stage are the four pneumatically operated bleed valves located on in the compressor bleed valve system?
Fifth stage
What triggers the speed sensitive valve in the compressor bleed valve system?
Engine RPM
What happens when the engine operates below 94% RPM in relation to the compressor bleed valve operation?
Compressor air forces open the fifth and tenth stage bleed valves
In the compressor bleed valve system, what role does the speed sensitive valve play when the engine is below 94% RPM?
It closes the bleed valves
What happens to the excess air when the bleed air valves are open in the compressor bleed valve system?
It is extracted out overboard
How does the speed sensitive valve respond to changes in engine RPM in the compressor bleed valve system?
By repositioning to control 5th and 10th stage air distribution
Study Notes
Compressor Bleed Valve System
- The compressor bleed valve system unloads the compressor during start and up to 13,000 RPM or 94% engine speed.
- The system unloads the compressor by bleeding air from the compressor's fifth and tenth stages.
- The engine can operate at low speed ground idle, reducing engine speed to save fuel and reduce noise during ground operations.
Speed Sensitive Valve
- The speed sensitive valve controls the compressor bleed valve system.
- The valve causes the compressor bleed valves to open if low speed ground idle or engine shutdown are selected.
- The speed sensitive valve repositions the speed sensing valve, opening the port which directs 14th stage air onto the bleed air valve piston heads when the engine is running at 94% RPM or more.
Compressor Bleed Valve Operation
- When the engine is running at 94% RPM or more, the compressor bleed air valves are forced closed.
- When the engine is operating at less than 94% RPM, the speed sensing valve flyweights position the sensing valve to stop 14th stage air acting on the 5th and 10th stage bleed valves, allowing compressor air to force open the 5th and 10th stage bleed valves.
Compressor Bleed Surge Control System
- Compressor bleed surge control systems are used on multi-spool engines.
- The system relieves the engine of excess air at low engine RPM during normal low speed operations, including engine start.
- When engine cranking first begins, air will flow inward through the compressor bleed valve to the inlet of the rear compressor, relieving part of the windmilling load of the front compressor.
Compressor Bleed Valve System Components
- The system consists of four pneumatically operated bleed valves located on the compressor fifth stage and tenth stage.
- The system also includes a speed sensitive valve mounted on the forward face of the accessory gearbox and all necessary plumbing and manifolds.
- The excess air is ducted overboard when the bleed air valves are open.
Test your knowledge on the operation of the aircraft bleed system at different RPM levels. Understand how the speed sensitive valves and air pressures determine the opening and closing of compressor bleed air valves.
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